llvm-6502/lib/CodeGen/PHIElimination.cpp
Chris Lattner 9e2dd8f8d7 Const correctness fixes
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7349 91177308-0d34-0410-b5e6-96231b3b80d8
2003-07-26 23:24:56 +00:00

256 lines
10 KiB
C++

//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
//
// This pass eliminates machine instruction PHI nodes by inserting copy
// instructions. This destroys SSA information, but is the desired input for
// some register allocators.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/CFG.h"
namespace {
struct PNE : public MachineFunctionPass {
bool runOnMachineFunction(MachineFunction &Fn) {
bool Changed = false;
// Eliminate PHI instructions by inserting copies into predecessor blocks.
//
for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
Changed |= EliminatePHINodes(Fn, *I);
//std::cerr << "AFTER PHI NODE ELIM:\n";
//Fn.dump();
return Changed;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addPreserved<LiveVariables>();
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
/// in predecessor basic blocks.
///
bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
};
RegisterPass<PNE> X("phi-node-elimination",
"Eliminate PHI nodes for register allocation");
}
const PassInfo *PHIEliminationID = X.getPassInfo();
/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
/// predecessor basic blocks.
///
bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
if (MBB.empty() || MBB.front()->getOpcode() != TargetInstrInfo::PHI)
return false; // Quick exit for normal case...
LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
while (MBB.front()->getOpcode() == TargetInstrInfo::PHI) {
MachineInstr *MI = MBB.front();
// Unlink the PHI node from the basic block... but don't delete the PHI yet
MBB.erase(MBB.begin());
assert(MI->getOperand(0).isVirtualRegister() &&
"PHI node doesn't write virt reg?");
unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
// Create a new register for the incoming PHI arguments
const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
// Insert a register to register copy in the top of the current block (but
// after any remaining phi nodes) which copies the new incoming register
// into the phi node destination.
//
MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
while (AfterPHIsIt != MBB.end() &&
(*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI)
++AfterPHIsIt; // Skip over all of the PHI nodes...
RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
// Update live variable information if there is any...
if (LV) {
MachineInstr *PHICopy = *(AfterPHIsIt-1);
// Add information to LiveVariables to know that the incoming value is
// killed. Note that because the value is defined in several places (once
// each for each incoming block), the "def" block and instruction fields
// for the VarInfo is not filled in.
//
LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy);
// Since we are going to be deleting the PHI node, if it is the last use
// of any registers, or if the value itself is dead, we need to move this
// information over to the new copy we just inserted...
//
std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
RKs = LV->killed_range(MI);
std::vector<std::pair<MachineInstr*, unsigned> > Range;
if (RKs.first != RKs.second) {
// Copy the range into a vector...
Range.assign(RKs.first, RKs.second);
// Delete the range...
LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
// Add all of the kills back, which will update the appropriate info...
for (unsigned i = 0, e = Range.size(); i != e; ++i)
LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
}
RKs = LV->dead_range(MI);
if (RKs.first != RKs.second) {
// Works as above...
Range.assign(RKs.first, RKs.second);
LV->removeVirtualRegistersDead(RKs.first, RKs.second);
for (unsigned i = 0, e = Range.size(); i != e; ++i)
LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
}
}
// Now loop over all of the incoming arguments, changing them to copy into
// the IncomingReg register in the corresponding predecessor basic block.
//
for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
MachineOperand &opVal = MI->getOperand(i-1);
// Get the MachineBasicBlock equivalent of the BasicBlock that is the
// source path the PHI.
MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
// Figure out where to insert the copy, which is at the end of the
// predecessor basic block, but before any terminator/branch
// instructions...
MachineBasicBlock::iterator I = opBlock.end();
if (I != opBlock.begin()) { // Handle empty blocks
--I;
// must backtrack over ALL the branches in the previous block
while (MII.isTerminatorInstr((*I)->getOpcode()) &&
I != opBlock.begin())
--I;
// move back to the first branch instruction so new instructions
// are inserted right in front of it and not in front of a non-branch
if (!MII.isTerminatorInstr((*I)->getOpcode()))
++I;
}
// Check to make sure we haven't already emitted the copy for this block.
// This can happen because PHI nodes may have multiple entries for the
// same basic block. It doesn't matter which entry we use though, because
// all incoming values are guaranteed to be the same for a particular bb.
//
// If we emitted a copy for this basic block already, it will be right
// where we want to insert one now. Just check for a definition of the
// register we are interested in!
//
bool HaveNotEmitted = true;
if (I != opBlock.begin()) {
MachineInstr *PrevInst = *(I-1);
for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
MachineOperand &MO = PrevInst->getOperand(i);
if (MO.isVirtualRegister() && MO.getReg() == IncomingReg)
if (MO.opIsDefOnly() || MO.opIsDefAndUse()) {
HaveNotEmitted = false;
break;
}
}
}
if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
assert(opVal.isVirtualRegister() &&
"Machine PHI Operands must all be virtual registers!");
unsigned SrcReg = opVal.getReg();
RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
// Now update live variable information if we have it.
if (LV) {
// We want to be able to insert a kill of the register if this PHI
// (aka, the copy we just inserted) is the last use of the source
// value. Live variable analysis conservatively handles this by
// saying that the value is live until the end of the block the PHI
// entry lives in. If the value really is dead at the PHI copy, there
// will be no successor blocks which have the value live-in.
//
// Check to see if the copy is the last use, and if so, update the
// live variables information so that it knows the copy source
// instruction kills the incoming value.
//
LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
// Loop over all of the successors of the basic block, checking to see
// if the value is either live in the block, or if it is killed in the
// block. Also check to see if this register is in use by another PHI
// node which has not yet been eliminated. If so, it will be killed
// at an appropriate point later.
//
bool ValueIsLive = false;
const BasicBlock *BB = opBlock.getBasicBlock();
for (succ_const_iterator SI = succ_begin(BB), E = succ_end(BB);
SI != E && !ValueIsLive; ++SI) {
const std::pair<MachineBasicBlock*, unsigned> &
SuccInfo = LV->getBasicBlockInfo(*SI);
// Is it alive in this successor?
unsigned SuccIdx = SuccInfo.second;
if (SuccIdx < InRegVI.AliveBlocks.size() &&
InRegVI.AliveBlocks[SuccIdx]) {
ValueIsLive = true;
break;
}
// Is it killed in this successor?
MachineBasicBlock *MBB = SuccInfo.first;
for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
if (InRegVI.Kills[i].first == MBB) {
ValueIsLive = true;
break;
}
// Is it used by any PHI instructions in this block?
if (ValueIsLive) break;
// Loop over all of the PHIs in this successor, checking to see if
// the register is being used...
for (MachineBasicBlock::iterator BBI = MBB->begin(), E=MBB->end();
BBI != E && (*BBI)->getOpcode() == TargetInstrInfo::PHI;
++BBI)
for (unsigned i = 1, e = (*BBI)->getNumOperands(); i < e; i += 2)
if ((*BBI)->getOperand(i).getReg() == SrcReg) {
ValueIsLive = true;
break;
}
}
// Okay, if we now know that the value is not live out of the block,
// we can add a kill marker to the copy we inserted saying that it
// kills the incoming value!
//
if (!ValueIsLive)
LV->addVirtualRegisterKilled(SrcReg, &opBlock, *(I-1));
}
}
}
// really delete the PHI instruction now!
delete MI;
}
return true;
}